In the CMS folder there is another folder called templates, inside of that template is a file called LeftAndMain.ss, if you look for the following code, you can either change the text at the bottom, or remove it completely.
<div id="bottom">
<div class="holder">
<div id="logInStatus">
<a href="http://www.silverstripe.com" title="<% _t('SSWEB','Silverstripe Website') %>">SilverStripe CMS</a> -
<abbr style="border-style: none" title="<% _t('APPVERSIONTEXT1',"This is the") %> $ApplicationName <% _t('APPVERSIONTEXT2',"version that you are currently running, technically it's the CVS branch") %>">$CMSVersion</abbr>
<% control CurrentMember %>
<% _t('LOGGEDINAS','Logged in as') %> <strong><% if FirstName && Surname %>$FirstName $Surname<% else_if FirstName %>$FirstName<% else %>$Email<% end_if %></strong> | <a href="{$BaseHref}admin/myprofile" id="EditMemberProfile"><% _t('EDITPROFILE','Profile') %></a> | <a href="Security/logout" id="LogoutLink"><% _t('LOGOUT','log out') %></a>
<% end_control %>
</div>
Hope that helps